Anode modification for organic light emitting diodes

ABSTRACT

An organic light emitting device is provided which includes a cathode ( 51 ), an anode ( 47, 46, 48 ), and an organic electroluminescent region ( 49, 50 ). The anode includes a metal layer ( 46 ), a barrier layer ( 47 ), and an anode modification layer ( 48 ). Light is emitted through the cathode ( 51 ) when a voltage is applied between the anode ( 47, 46, 48 ) and the cathode ( 51 ).

TECHNICAL FIELD

The present invention pertains to organic electroluminescent displaysand methods for making the same.

BACKGROUND OF THE INVENTION

Organic electroluminescence (EL) has been studied extensively because ofits possible applications in discrete light emitting diodes (LED),arrays and displays. Organic materials can potentially replacesemiconductors in many LED applications and enable wholly newapplications. The ease of organic LED (OLED) fabrication and thecontinuing development of improved organic materials promise novel andinexpensive OLED display possibilities.

Organic EL at low efficiency was reported many years ago in e.g.Helfrich et al., Physical Review Letters, Vol. 14, No. 7, 1965, pp.229-231. Recent developments have been spurred by two reports ofefficient organic EL: C. W. Tang et al., Applied Physics Letters, Vol.51, No. 12, 1987, pp. 913-915, and Burroughes et al., Nature, Vol. 347,1990, pp. 539. Tang used vacuum deposition of molecular compounds toform OLEDs with two organic layers. Burroughes spin coated a polymer,poly(p-phenylenevinylene), to form a single-organic-layer OLED. Theadvances described by Tang and in subsequent work by N. Greenham et al.,Nature, Vol. 365, 1993, pp. 628-630, were achieved mainly throughimprovements in OLED design derived from the selection of appropriateorganic multilayers and electrode metals.

The simplest possible OLED structure, depicted in FIG. 1A, consists ofan organic emission layer 10 sandwiched between cathode 11 and anode 12electrodes which inject electrons (e⁻) and holes (h⁺), respectively,which meet in the emission layer 10 and recombine producing light. Ithas been shown (D. D. C. Bradley, Synthetic Metals, Vol. 54, 1993, pp.401-405, J. Peng et al., Japanese Journal of Applied Physics, Vol. 35,No. 3A, 1996, pp. L317-L319, and I. D. Parker, Journal of AppliedPhysics, Vol. 75, No. 3, 1994, pp. 1656-1666) that improved performanceis achieved if the electrode work functions match the respectivemolecular orbitals (MO) of the organic layer 10. Such an improvedstructure is shown in FIG. 1B where optimized electrode materials 13 and14 reduce the energy barriers to carrier injection into the organiclayer 10. Still, single organic layer structures perform poorly becauseelectrons can traverse the organic layer 10 reaching the anode 14, orholes may reach the cathode 13, in either case resulting in currentwithout light, and lower OLED efficiency.

Balanced charge injection is also important. For example, an excellentanode is of limited use if the cathode has a large energy barrier toelectron injection. FIG. 2 illustrates a device with a large electronbarrier 16 such that few electrons are injected, leaving the holes nooption but to recombine non-radiatively in or near the cathode 15. Theanode and cathode materials should be evenly matched to their respectiveMOs to provide balanced charge injection and optimized OLED efficiency.

An improved structure in which the electron and hole transport functionsare divided between separate organic layers, an electron transport layer20 (ETL) and a hole transport layer (HTL) 21, is shown in FIG. 3. In C.W. Tang et al., Journal of Applied Physics, Vol. 65, No. 9, 1989; pp.3610-3616, it is described that higher carrier mobility is achieved in atwo organic layer OLED design, and this led to reduced OLED seriesresistance enabling equal light output at lower operating voltage. Theelectrodes 22, 23 can be chosen individually to match to the ETL 20 andHTL 21 MOs, respectively, while recombination occurs at the interface 24of organic layers 20 and 21, far from either electrode. As electrodes,Tang used a MgAg alloy cathode and transparent Indium-Tin-Oxide (ITO)deposited on a glass substrate as the anode. Egusa et al. in JapaneseJournal of Applied Physics, Vol. 33, No. 5A, 1994, pp. 2741-2745 haveshown that proper selection of the organic multilayer materials leads toenergy barriers blocking both electrons and holes at the organicinterface. This is illustrated in FIG. 3 in which electrons are blockedfrom entering the HTL 21 and holes from entering the ETL 20 by a cleverchoice of organic materials. This feature reduces quenching near thecontacts (as illustrated in FIG. 2) and also promotes a high density ofelectrons and holes in the small interface volume providing enhancedradiative recombination.

With multilayer device architectures now well understood and widelyused, a remaining performance limitation of OLEDs is the electrodes. Themain figure of merit for electrode materials is the position of theelectrode Fermi energy relative to the relevant organic MO. In someapplications it is also desirable for an electrode to be eithertransparent or highly reflective to assist light extraction. Electrodesmust also be chemically inert with respect to the adjacent organicmaterial to provide long term OLED stability.

Much attention has been paid to the cathode, largely because goodelectron injectors are low work function metals which are. alsochemically reactive and oxidize quickly in atmosphere, limiting the OLEDreliability and lifetime. Much less attention has been paid to theoptimization of the anode contact, since conventional ITO anodesgenerally outperform the cathode contact leading to an excess of holes.Due to this excess, and the convenience associated with the transparencyof ITO, improved anodes have not been as actively sought as improvedcathodes.

ITO is by no means an ideal anode, however. ITO is responsible fordevice degradation as a result of In diffusion into the OLED eventuallycausing short circuits as identified by G. Sauer et al., Fresenius J.Anal. Chem., pp. 642-646, Vol. 353 (1995). ITO is polycrystalline andits abundance of grain boundaries provides ample pathways forcontaminant diffusion into the OLED. Finally, ITO is a reservoir ofoxygen which is known to have a detrimental effect on many organicmaterials (see J. C. Scott, J. H. Kaufman, P. J. Brock, R. DiPietro, J.Salem, and J. A. Goita, J. Appl. Phys., Vol. 79, p. 2745, 1996). Despiteall of these problems, ITO anodes are favored because no bettertransparent electrode material is known and ITO provides adequatestability for many applications.

While conventional OLEDs extract light through the ITO anode,architectures relying on light extraction through a highly transparentcathode (TC) are desirable for transparent OLEDs or OLEDs fabricated onan opaque substrate. Si is an especially desirable OLED substratebecause circuits fabricated in the Si wafer can be cheaply integratedwith drive circuitry providing display functions. Given theminaturization and outstanding performance of Si circuitry, a highinformation content OLED/Si display could be inexpensively fabricated ona Si integrated circuit (IC).

The simplest approach incorporating a TC is to deposit a thin,semi-transparent low work function metal layer, e.g. Ca or MgAg,followed by ITO or another transparent, conducting material ormaterials, e.g. as reported in Bulovic et al., Nature, Vol. 380, No. 10,1996 p. 29, or in the co-pending PCT patent application PCT/IB96/00557,published on Dec. 11, 1997 (publication number W097/47050). To maximizethe efficiency of such a TC OLED, a highly reflective anode which candirect more light out through the TC is desired. Consequently, the lowreflectivity of ITO is a disadvantage in TC OLEDs.

Alternatively, for some applications it may be more important toincrease the contrast ratio of the OLEDs or display based thereon. Inthis case, a TC OLED could benefit from a non-reflective, highlyabsorbing anode. Again the optical characteristics of ITO are adisadvantage.

High work function metals could form highly reflective anodes for TCOLEDs. Some of these metals, e.g. Au, have a larger work function thanITO (5.2 eV vs. 4.7 eV), but lifetime may be compromised because of highdiffusivity in organic materials. Like In from ITO, only worse, Audiffuses easily through many organic materials and can eventually shortcircuit the device.

Efforts have been made to fabricate OLEDs on Si substrates (Parker andKim, Applied Physics Letters, Vol. 64, No. 14, 1994, pp. 1774-1776). Si,due to its small bandgap and moderate work function, has a large barrierfor both electron and hole injection into organic MOs, and thereforeperforms poorly as an electrode. Parker and Kim improved the situationsomewhat by adding a SiO₂ interlayer between the Si and OLED. A voltagedrop across the SiO₂ insulator permitted the Si bands to line up withtheir organic MO counterpart and electrons or holes from the Si totunnel through the SiO₂ into the organic MO. However, the requiredvoltage drop across the SiO₂ raised the OLED turn-on voltage >10 V,making these OLEDs inefficient. Low voltage OLED/Si designs aredesirable not just to improve efficiency, but also to facilitate circuitdesign since sub-micron Si transistors cannot easily produce drivevoltages >10 V. For anodes, more desirable than a tunneling insulatorsurface modification like SiO₂ is one which raises the Si surface workfunction thereby lowering the OLED operating voltage.

As can be seen from the above examples and description of the state ofthe art, electrode materials must be improved to realize OLEDs, anddisplays based thereon, with superior reliability and efficiency, and toenable novel architectures, such as devices emitting through a TC. Inparticular, to fabricate an OLED array or display on a Si substrate, animproved anode compatible with Si IC technology is required foroptimized TC OLED architectures.

It is an object of the present invention to provide new and improvedorganic EL devices, arrays and displays based thereon.

It is a further purpose of the present invention to provide new andimproved organic EL devices, arrays and displays based thereon optimizedfor light emission through a transparent cathode electrode with improvedefficiency, lower operating voltage, or steeper current/voltagecharacteristic and increased reliability, stability and lifetime.

It is another object of the present invention to provide new andimproved anodes for organic EL devices, arrays and displays fabricatedon Si substrates.

It is a further object to provide a method for making the present newand improved organic EL devices, arrays and displays.

SUMMARY OF THE INVENTION

The above objects have been accomplished by providing an OLED having acathode, an anode, and an organic region sandwiched in between, saidanode being composed of

a metal layer,

an anode modification layer, and

at least one barrier layer,

said anode being arranged such that said anode modification layer is incontact with said organic region and light is extracted through saidcathode.

Any kind of metal is suited as metal layer in connection with thepresent invention. Examples are Al, Cu, Mo, Ti, Pt, Ir, Ni, Au, Ag, andany alloy thereof, or any metal stack such as Pt on Al and the like.

The inventive approach is specifically designed for the fabrication ofOLEDs on top of Si, preferably Si crystalline wafers incorporatingpre-processed integrated display circuitry (herein referred to as SiIC). The present invention is designed to modify the existing Si devicemetallization into a stable OLED anode having good hole injectionproperties. For OLEDs on top of a Si IC, the metal layer in the presentinvention is generally the final metallization layer of the Si ICprocess, which consistent with present Si technology is normally Al, Cuor an alloy thereof. Neither Al, Cu or Al:Cu alloys perform well as OLEDanodes, but they do provide excellent visible spectrum reflectivitywhich increases the amount of light extracted through a TC. The Si ICmetallization surface can vary widely in terms of oxide thickness,roughness and surface contaminants depending on numerous factors,including the fabrication process, the time between IC fabrication andOLED deposition, and the environment in which the Si IC was stored andshipped. For reproduceable fabrication of efficient OLEDs the Simetallization anode properties must be improved and effects arising fromvariations of the initial state of the metal surface must be eliminated.

The inventive approach is also suited for use with pixel and drivecircuitry comprising polysilicon or amorphous silicon devices.

The anode modification layer in the present invention is mainly selectedfor its high work function which provides efficient hole injection intoOLEDs. The anode modification layer must form a stable interface withthe adjacent organic layer being part of the so-called organic region(e.g. the organic HTL) to insure consistent OLED performance over anextended time period. The anode modification layer can be conductive orinsulating, but it should be sufficiently thin that it contributesnegligibly both to the OLED series resistance and optical absorptionlosses. Oxides are well suited as anode modification layers. Thethickness of the anode modification layer is preferably between 0.5 nmand 10 nm.

The barrier layer or layers in the present invention isolates the anodemodification layer from the metal layer by forming a physical andchemical barrier, while permitting charge to pass freely through itsinterfaces with the metal layer and anode modification layer. Thebarrier layer(s) provides a consistent and reproduceable surface for thedeposition or formation of the anode modification layer regardless ofthe metal layer composition or initial state of its surface. The barrierlayer(s) can be conductive or insulating, but it (they) should besufficiently thin that it (they) contributes negligibly to the OLEDseries resistance. Alternatively, the barrier layer(s) can be highlyreflective which avoids absorption losses. The thickness of the barrierlayer is preferably between 5 nm-100 nm. Well suited are barrier layerscomprising TiN or TiNC, for example.

For formation on a Si wafer, all of the layers comprising the anode mustbe depositable or formable onto the wafer using processes sufficientlygentle that underlying Si circuitry is undamaged, i.e. at lowtemperature causing little chemical or physical damage.

In one embodiment of the present invention, a single or multilayer OLEDstructure having a TC fabricated on a Si substrate incorporates amultilayer anode structure comprising a metal layer, an anodemodification layer, and an intermediate barrier layer(s), such that theanode is stable and efficient at hole injection.

The introduction of such an anode into the OLED structure leads to thefollowing advantages:

Low voltage hole injection via both the high work function of the anodemodification layer and the free passage of charge from the metal layer,through the barrier layer(s) into the anode modification layer.

Stable OLED operation over an extended time period via the chemical andphysical barrier the barrier layer(s) provides between the metal layerand anode modification layer, and the stability of the anodemodification layer interface with the adjacent organic HTL.

Efficient light extraction through the TC aided by the high reflectivityand low absorption of the metal layer, barrier layer(s) and anodemodification layer stack.

DESCRIPTION OF THE DRAWINGS

The invention is described in detail below with reference to thefollowing schematic drawings. It is to be noted that the Figures are notdrawn to scale.

FIG. 1A shows the band structure of a known OLED having an emissionlayer and two electrodes (Prior Art).

FIG. 1B shows the band structure of another known OLED having anemission layer and two metal electrodes, with work functions chosen suchthat the energy barrier for carrier injection is reduced (Prior Art).

FIG. 2 shows the band structure of another known OLED having an emissionlayer and two metal electrodes, the work function of the anode beingchosen such that the energy barrier for hole injection is low, whereasthe work function of the cathode poorly matches the emission layeryielding little electron injection and little radiative recombination insaid emission layer (Prior Art).

FIG. 3 shows the band structure of another known OLED having an electrontransport layer and hole transport layer (Prior Art).

FIG. 4 is a schematic cross section of a first embodiment of the presentinvention.

FIG. 5 is a schematic cross section of a second embodiment of thepresent invention.

FIG. 6 is a schematic cross section of a third embodiment of the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

A first improved structure benefiting from the inventive anode approachis illustrated and discussed in connection with FIG. 4.

To provide high performance TC OLED devices fabricated on Si substrates,improved structures benefiting from the inventive anode approach, asillustrated in FIGS. 4-6, are provided, enabling new array and displayapplications. Three embodiments of improved OLEDs incorporating theinventive anodes are detailed in connection with FIGS. 4-6.

A first embodiment is depicted in FIG. 4. A TC OLED is shown which isformed on a substrate 45. Since in the present configuration theelectroluminescent light 52 is emitted through the top electrode(cathode 51), almost any kind of substrate 45 can be used. Examples areSi, glass, quartz, stainless steel, and various plastics. The inventiveanode, comprising a metal layer 46, a barrier layer 47, and an anodemodification layer 48 is situated on said substrate 45. Any kind ofmetal is suited as metal layer 46. Examples are Al, Cu, Mo, Ti, Pt, Ir,Ni, Au, Ag, and any alloy thereof, or any metal stack such as Pt on Aland the like. Depending on the embodiment, particularly well suited aremetals which provide visible spectrum reflectivity.

The barrier layer 47 isolates the anode modification layer 48 from themetal layer 46 by forming a physical and chemical barrier, whilepermitting charge to pass freely through its interfaces with the metallayer 46 and anode modification layer 48. Note that the inventive anodemight comprise one or more barrier layers. The barrier layer(s) 47provides a consistent and reproduceable surface for the formation ordeposition of the anode modification layer 48 regardless of the metallayer composition or initial state of its surface. The barrier layer(s)47 can be conductive or insulating, but it (they) should be sufficientlythin that it (they) contributes negligibly to the OLED seriesresistance. Alternatively, the barrier layer(s) 47 can be reflectivewhich avoids absorption losses. The thickness of the barrier layer ispreferably between 5 nm-100 nm.

The anode modification layer 48 is mainly selected for its high workfunction which provides efficient hole injection into the organic regionof the OLED. The anode modification layer 48 must form a stableinterface with the adjacent organic emission layer (EML) 49 to insure.consistent OLED performance over an extended time period. The anodemodification layer 48 can be conductive or insulating, but it must besufficiently thin that it contributes negligibly to both the OLED seriesresistance and the optical absorption losses. The thickness of the anodemodification layer is preferably between 0.5 nm and 10 nm.

A TC 51 is situated on the EML 49. The electroluminescence takes placewithin the EML 49. As indicated in FIG. 4, part of the light is emitteddirectly through the EML 49 and the TC 51 into the half space above theOLED. Another part of the light travels towards the inventive anodestructure. The anode structure reflects the light such that it is alsoemitted into the half space above the OLED.

TABLE 1 Exemplary details of the first embodiment present Layer No.Material Thickness example Substrate 45 Quartz 0.05-5 mm 800 μm MetalLayer 46 Ti/Al 0.01-0.7 μm/ 2 nm 0.05-3 μm Barrier Layer 47 Ni 0.01-2 μm30 nm Anode modification 48 ITO 0.003-2 μm 7 nm Layer Emission Layer 49PPV 50-500 nm 200 nm Transparent Cathode 51 Li:Al alloy 50-1000 nm 120nm

A second embodiment of the inventive anode for TC OLEDs fabricated on Sisubstrates is depicted in FIG. 5. From the substrate 25 up, listed inthe order of deposition, is a Si IC/InN/InNO_(x)/HTL/ETL/TC OLEDstructure. The Si IC 25 in FIG. 5 comprises an Al top metal contact pad26 which also serves as the metal layer in the inventive anode. Aftercompletion of the Si IC fabrication, an InN barrier layer 27 isdeposited directly onto the substrate such that it overlaps at least thecontact pad 26. The sample is then oxidized in an oxygen plasma, orequivalently in an air, steam, ozone or other oxidizing environment toprepare an InNO_(x) surface anode modification layer 28 capable of lowvoltage hole injection into the organic HTL 29. Electrons injected intothe ETL 30 from the TC 31 recombine with the holes in the organic regionproducing EL 32 which is extracted through the TC 31. The organic regionin the present embodiment comprises an HTL 29 and an ETL 30. It is to benoted that the organic region in any case at least comprises one organiclayer (see first embodiment, for example). The Si IC 25 might compriseintegrated circuitry which is not illustrated in FIG. 5, for sake ofsimplicity. Instead of InN other group III nitrides might be used, forexample.

InN is an excellent barrier layer material because it is a degeneratesemiconductor which has both excellent transparency and is conductive,but not so conductive that lateral conduction through the InN barrierlayer 27 between adjacent Al metal pads 26 on the Si IC 25 causeselectrical crosstalk. InN having these properties can be deposited at ornear room temperature as described in Beierlein et al., MaterialsResearch Society Internet Journal of Nitride Semiconductor Research,Vol. 2, Paper 29. InN is also a convenient choice because its surfacework function can be increased by oxidation thus forming an InNO_(x)anode modification layer 28 directly on the InN barrier layer.Equivalently, the InNO_(x) anode modification layer 28 could be directlydeposited onto the InN.

Several methods of deposition of oxide-based anode modification layersare listed below:

chemical vapor deposition (CVD), including plasma-enhanced chemicalvapor deposition (PECVD);

sputter deposition or reactive (e.g. in an oxygen environment) sputterdeposition;

thermal evaporation;

electron-beam evaporation;

oxygen plasma (plasma-assisted oxidation);

thermal annealing in an oxidizing environment;

UV-ozone treatment;

wet-chemical oxidation;

electrochemical oxidation;

spin-coating from solution.

A completely different anode modification layer 28 could also besubstituted, e.g. ITO, ZnO, MgO, Sn₂O₃, In₂O₃, RuO₂, or V₂O₅, or similaroxides, just to give some examples. Similarly, a completely differentmetal layer 26 could also be substituted. The degeneracy of the InNsemiconductor insures that charge can pass easily from the metal layer26 into the InN barrier layer 27, regardless of the composition orinitial state of the metal layer 26 surface. For the same reason, chargecan also traverse the InN barrier layer 27 and into the anodemodification layer 28 without significant series resistance. Thestrength of the highly polar In—N bond insures that InN 27 acts as anexcellent chemical and physical barrier to corrosion or diffusionbetween the metal layer 26 and the anode modification layer 28.

The device depicted in FIG. 5 benefits from the high reflectivity of theAl metal layer 26, and the low visible spectrum absorption of the InNbarrier layer 27 and the InNO_(x) anode modification layer 28, whichpermits much of the EL 32 emitted towards the substrate to be reflectedback through the TC 31.

Devices having the anode structure depicted in FIG. 5 exhibit higherquantum and power efficiencies than comparable structures havingconventional ITO anodes as a result of more balanced charge injectionbetween the Al/InN/InNO_(x) anode and the TC.

TABLE 2 Exemplary details of the second embodiment present Layer No.Material Thickness example Substrate 25 Si IC 0.05-1 mm 500 μm MetalLayer 26 Aluminum 0.05-2 μm 500 nm Barrier Layer 27 InN 0.01-1 μm 20 nmAnode 28 InNO_(x) 0.001-1 μm 5 mn modification Layer Hole Transport 29TAD 10-200 nm 50 nm Layer Electron 30 Alq₃ 10-200 nm 50 nm TransportLayer Transparent 31 MgAg/ITO 1-20 nm/ 80 nm/ Cathode 10-1000 nm 50 nm

A third embodiment of the inventive anode for TC OLEDs fabricated on Sisubstrates is depicted in FIG. 6. From the substrate 33 up, listed inthe order of deposition, is a Si IC/Ni/NiO_(x)/V₂O₅/HTL/EML/ETL/TC OLEDstructure. The Si IC 33 in FIG. 6 comprises an Al:Cu alloy top metalcontact 34 which serves as the metal layer in the inventive anode. Aftercompletion of the Si IC, a two layer Ni 35/NiO_(x) 36 barrier layer isdeposited in sequence directly onto those parts of the substrate 33which are covered by the metal contact layer 34, or alternatively, theNi 35 barrier layer is deposited, and its surface is subsequentlyoxidized to form the NiO_(x) 36 barrier layer. The inventive anode iscompleted by the deposition of the V₂O₅ anode modification layer 37capable of injecting holes into the HTL 38. Electrons injected into theETL 40 from the TC 41 recombine in the organic emission layer 39producing EL 42 which is extracted through the TC 41. The circuitry inthe Si substrate is not shown for simplicity.

We note that an arbitrarily large number of barrier layers 35, 36 can beinserted between the metal layer 34 and the anode modification layer 37of the inventive anode provided that they do not reduce deviceefficiency through excessive series resistance at their interfaces or intheir bulk. The Ni 35/NiO_(x) 36 barrier layer structure of the presentembodiment relies on the high reflectivity of the Ni 35 metal and thetransparency and thinness of the insulating NiO_(x) 36 layer to insuregood device efficiency.

We note that the third embodiment without the Ni and NiO_(x) layers isunstable due to a chemical reaction between the Al:Cu alloy 34 and theanode modification layer 37. The oxidation of the Ni 35 surface, oralternatively the deposition of an additional barrier layer furtherchemically isolates the Ni metal from the anode modification layer 37.Because the Ni 35 barrier layer is highly conductive, it must be verythin, or patterned (as shown in FIG. 6) to avoid lateral conductionbetween adjacent IC metallizations 34 (not shown in FIG. 6).

Devices having the inventive metal/Ni/NiO_(x)/V₂O₅ anode structuredepicted in FIG. 6 exhibit steeper current/voltage characteristics thanconventional ITO anodes and similar power efficiencies.

TABLE 3 Exemplary details of the third embodiment present Layer No.Material Thickness example Substrate 33 Si IC 0.05-5 mm 500 μm MetalLayer 34 Al-Cu 0.05-5 μm 500 nm Barrier Layer 35 Ni 0.1-1000 nm 2.5 nmBarrier Layer 36 NiO_(x) 0.1-10 nm 1 nm Anode 37 V₂O₅ 0-1 μm 5 nmmodification layer Hole Transport 38 NPB 10-200 nm 50 nm Layer EmissionLayer 39 Alq₃: 1-100 nm 15 nm Rubrene Electron 40 Alq₃ 10-200 nm 50 nmTransport Layer Transparent 41 Ca/InGaN/ 1-20 nm/ 10 nm/ Cathode ITO10-100 nm/ 80 nm/ 10-1000 nm 150 nm

In the following, some display embodiments, based on and enabled by thepresent invention, are addressed.

Any of the three embodiments, or modifications thereof, can be part of adisplay or array. The Si substrate, for example, might compriseintegrated circuitry which drives the pixels of the OLEDs formedthereon. For this purpose, the inventive anode might be connected to themetal contact of an active matrix element formed in the Si IC substrate.If the circuitry is patterned appropriately, individual pixels or groupsof pixels can be turned on and off.

Arrays and displays can be realized with high quantum and powerefficiencies, lower threshold voltages, and/or steep current/voltagecharacteristics. The inventive anode is compatible with many knownapproaches.

It is advantageous to integrate OLEDs onto Si substrates because priorto OLED deposition, the substrate can be fabricated to contain theactive Si devices, such as for example an active matrix, drivers, memoryand so forth. Such an OLED on Si structure can be a very inexpensivesmall area organic display with high resolution and performance. An OLEDor OLED array may either be grown directly on such a Si substratecarrying Si devices, or it may be fabricated separately and assembled ina flip-chip fashion onto the Si circuitry later.

One problem is that the Si metallization is typically Al or an Al alloywhich are poor OLED anode or cathode materials. The inventive anodepermits a stable, low voltage hole contact to be formed on top of thestandard Si process metallizations.

In the following some examples of the different organic materials whichcan be used are given.

Electron Transport/Emitting Materials

Alq₃, Gaq₃, Inq₃, Scq₃, (q refers to 8-hydroxyquinolate or it'sderivatives) and other 8-hydroxyquinoline metal complexes such as Znq₂,Beq₂, Mgq₂, ZnMq₂, BeMq₂, BAlq, and AlPrq₃, for example. These materialscan be used as the ETL or emission layer.

Other classes of electron transporting materials are electron-deficientnitrogen-containing systems, for example oxadiazoles like PBD (and manyderivatives), and triazoles, for example TAZ (1,2,4-triazole).

These functional groups can also be incorporated in polymers, starburstand spiro compounds. Further classes are materials containing pyridine,pyrimidine, pyrazine and pyridazine functionalities.

Finally, materials containing quinoline, quinoxaline, cinnoline,phthalazine and quinaziline functionalities are well known for theirelectron transport capabilities.

Other materials are didecyl sexithiophene (DPS6T), bis-triisopropylsilylsexithiophene (2D6T), azomethin-zinc complexes, pyrazine (e.g. BNVP),styrylanthracene derivatives (e.g. BSA-1, BSA-2), non-planardistyrylarylene derivatives, for example DPVBi (see C. Hosokawa and T.Kusumoto, International Symposium on Inorganic and OrganicElectroluminescence 1994, Hamamatsu, 42), cyano-substituted-polymerssuch as cyano-PPV (PPV means poly(p-phenylenevinylene)) and cyano-PPVderivatives.

The following materials are particularly well suited as

Emission Layers and Dopants

Anthracene, pyridine derivatives (e.g. ATP), Azomethin-zinc complexes,pyrazine (e.g. BNVP), styrylanthracene derivatives (e.g. BSA-1, BSA-2),Coronene, Coumarin, DCM compounds (DCM1, DCM2), distyryl arylenederivatives (DSA), alkyl-substituted distyrylbenzene derivatives (DSB),benzimidazole derivatives (e.g. NBI), naphthostyrylamine derivatives(e.g. NSD), oxadiazole derivatives (e.g. OXD, OXD-1, OXD-7),N,N,N′,N′-tetrakis(m-methylphenyl)-1,3-diaminobenzene (PDA), peryleneand perylene derivatives, phenyl-substituted cyclopentadienederivatives, 12-phthaloperinone sexithiophene (6T), polythiophenes,quinacridones (QA) (see T. Wakimoto et al., International Symposium onInorganic and Organic Electroluminescence, 1994, Hamamatsu, 77), andsubstituted quinacridones (MQA), rubrene, DCJT (see for example: C.Tang, SID Conference San Diego; Proceedings, 1996, 181), conjugated andnon-conjugated polymers, for example PPV and PPV derivatives, dialkoxyand dialkyl PPV derivatives, for example MEH-PPV(poly(2-methoxy)-5-(2′-ethylhexoxy)-1,4-phenylene-vinylene),poly(2,4-bis(cholestanoxyl)-1,4-phenylene-vinylene) (BCHA-PPV), andsegmented PPVs (see for example: E. Staring in International Symposiumon Inorganic and Organic Electroluminescence, 1994, Hamamatsu, 48, andT. Oshino et al. in Sumitomo Chemicals, 1995 monthly report).

Hole Transport Layers and Hole Injection Layers

The following materials are suited as hole injection layers and holetransport layers. Materials containing aromatic amino groups, liketetraphenyldiaminodiphenyl (TPD-1, TPD-2, or TAD) and NPB (see C. Tang,SID Meeting San Diego, 1996, and C. Adachi et al. Applied PhysicsLetters, Vol. 66, p. 2679, 1995), TPA, NIPC, TPM, DEH (for theabbreviations see for example: P. Borsenberger and D. S. Weiss, OrganicPhotoreceptors for Imaging Systems, Marcel Dekker, 1993). These aromaticamino groups can also be incorporated in polymers, starburst (forexample: TCTA, m-MTDATA, see Y. Kuwabara et al., Advanced Materials, 6,p. 677, 1994, Y. Shirota et al., Applied Physics Letters, Vol. 65, p.807, 1994) and spiro compounds.

Further examples are: Copper(II) phthalocyanine (CuPc),(N,N′-diphenyl-N,N′-bis-(4-phenylphenyl)-1,1′-biphenyl-4,4′-diamine),distyryl arylene derivatives (DSA), naphthalene, naphthostyrylaminederivatives (e.g. NSD), quinacridone (QA), poly(3-methylthiophene)(P3MT) and its derivatives, perylene and perylene derivatives,polythiophene (PT), 3,4,9,10-perylenetetracarboxylic dianhydride(PTCDA), PPV and some PPV derivatives, for example MEH-PPV,poly(9-vinylcarbazole) (PVK), discotic liquid crystal materials (HPT).

There are many other organic materials known as being good lightemitters, charge transport materials, and charge injection materials,and many more will be discovered. These materials can be used as wellfor making light emitting structures according to the present invention.More information on organic materials can be found in text books andother well known publications, such as the book “Inorganic and OrganicElectroluminescence”, edited by R. H. Mauch et al., Wissenschaft undTechnik Verlag, Berlin, Germany, 1996, and the book “1996 SIDInternational Symposium, Digest of Technical Papers”, first edition,Vol. XXVII, May 1996, published by Society for Information Display, 1526Brookhollow Dr., Suite 82, Santa Ana, Calif., USA.

Additionally, blend (i.e. guest-host) systems containing active groupsin a polymeric binder are also possible. The concepts employed in thedesign of organic materials for OLED applications are to a large extentderived from the extensive existing experience in organicphotoreceptors. A brief overview of some organic materials used in thefabrication of organic photoreceptors is found in the above mentionedpublication of P. Brosenberger and D. S. Weiss, and in Teltech,Technology Dossier Service, Organic Electroluminescence (1995), as wellas in the primary literature.

OLEDs have been demonstrated using polymeric, oligomeric and smallorganic molecules. The devices formed from each type of molecule aresimilar in function, although the deposition of the layers varieswidely. The present invention is equally valid in all forms describedabove for organic light emitting devices based on polymers, oligomers,or small molecules.

Small molecule devices are routinely made by vacuum evaporation. This iscompatible with the process used for the formation of the inventiveanode.

Evaporation can be performed in a Bell jar type chamber withindependently controlled resistive and electron-beam heating of sources.It can also be performed in a molecular beam deposition systemincorporating multiple effusion cells and sputter sources. Oligomericand polymeric organics can also be deposited by evaporation of theirmonomeric components with later polymerization via heating or plasmaexcitation at the substrate. It is therefore possible to copolymerize orcreate mixtures by co-evaporation.

In general, polymer containing devices (single layer, multilayer,polymer blend systems, etc.) are made by dissolving the polymer in asolvent and spreading it over the substrate either by spin coating orthe doctor blade technique. After coating the substrate, the solvent isremoved by evaporation or otherwise. This method allows the fabricationof well defined multilayer organic stacks, provided that the respectivesolvents for each subsequent layer do not dissolve previously depositedlayers.

Additionally, hybrid devices containing both polymeric and evaporatedsmall organic molecules are possible. In this case, the polymer film isgenerally deposited first, since evaporated small molecule layers oftencannot withstand much solvent processing.

What is claimed is:
 1. An organic electroluminescent device comprisingan anode, a cathode, and an organic region sandwiched between said anodeand said cathode, said anode further comprising: a metal layer, abarrier layer, and an anode modification layer, wherein said anode isarranged such that said anode modification layer is in contact with saidorganic region and light is emitted through said cathode.
 2. The deviceof claim 1, wherein the surface of the barrier layer is oxidized to formthe anode modification layer.
 3. The device of claim 1, wherein theanode modification layer is deposited onto the barrier layer.
 4. Thedevice of claim 1, wherein the barrier layer comprises a group IIInitride.
 5. The device of claim 1, wherein the group III nitride is InN.6. The device of claim 1, wherein the metal layer comprises Al or Cu. 7.The device of claim 1, wherein the anode modification layer has a highworkfunction.
 8. The device of claim 1, wherein the metal layercomprises metal which provides visible spectrum reflectivity toincreases the amount of light extracted through said cathode.
 9. Thedevice of claim 1, wherein the anode modification layer and/or thebarrier layer strongly absorb visible light.
 10. The device of claim 1,wherein the metal layer comprises a metal selected from the groupconsisting of Al, Cu, Mo, Ti, Pt, Ir, Ni, Au, and Ag.
 11. The device ofclaim 1, wherein the metal layer comprises an alloy of a metal, saidmetal being selected from the group consisting of Al, Cu, Mo, Ti, Pt,Ir, Ni, Au and Ag.
 12. The device of claim 1, wherein the metal layercomprises a stack of a first metal over a second metal, said first andsecond metals being selected from the group consisting of Al, Cu, Mo,Ti, Pt, Ir, Ni, Au and Ag.
 13. The device of claim 1, wherein the metallayer is formed on a substrate.
 14. The device of claim 13, wherein thesubstrate is a silicon substrate.
 15. The device of claim 14, whereinthe silicon substrate is crystalline.
 16. The device of claim 13,wherein the substrate comprises polysilicon circuitry or amorphouscircuitry.
 17. The device of claim 14, wherein the silicon substratecomprises integrated circuitry.
 18. The device of claim 1, wherein theanode modification layer is adapted to provide efficient hole injectioninto the organic region.
 19. The device of claim 1, wherein the anodemodification layer is adapted to form a stable interface with theorganic region.
 20. The device of claim 1, wherein the anodemodification layer is conductive or insulating.
 21. The device of claim1, wherein an oxide layer serves as anode modification layer.
 22. Thedevice of claim 1, wherein the barrier layer physically and chemicallyisolates the anode modification layer from the metal layer by forming abarrier, while permitting charge to pass through its interfaces with themetal layer and anode modification layer.
 23. The device of claim 1,wherein the barrier layer is conductive or insulating.
 24. The device ofclaim 1, wherein the barrier layer is highly reflective.
 25. The deviceof claim 1, wherein the anode modification layer comprises an oxideselected from the group consisting of ITO, ZnO, MgO, Sn₂O₃, In₂O₃, RuO₂,and V₂O₅.
 26. The device of claim 1, wherein the barrier layer comprisesa Ni layer and a NiO_(x) layer, and the anode modification layercomprises V₂O₅.
 27. A display apparatus comprising a substrate and anarray of organic electroluminescent devices formed on said substrate,each one of said organic electroluminescent devices comprising an anode,a cathode and an organic region sandwiched between said anode and saidcathode, said anode further comprising a metal layer, a barrier layer,and an anode modification layer, wherein said anode is arranged suchthat said anode modification is in contact with said organic region andlight is emitted through said cathode.
 28. The display apparatus ofclaim 27, wherein said substrate is a silicon substrate.
 29. The displayapparatus of claim 28, wherein circuitry is integrated into thesubstrate.
 30. The display apparatus of claim 29, wherein the circuitryis adapted to drive the device.
 31. The display apparatus of claim 27,wherein said anode is patterned.
 32. A method for making an organicelectroluminescent device comprising an anode, a cathode, and an organicregion being sandwiched between said anode and said cathode, said methodcomprising a step of building up said anode prior to the formation ofsaid organic region and cathode, by forming a metal layer, a barrierlayer situated on the metal layer, and an anode modification layersituated on the barrier layer, said anode being arranged such that saidanode modification layer is in contact with said organic region andlight is emitted through said cathode.
 33. The method of claim 32,wherein an oxide serves as said anode modification layer, said oxidebeing formed by oxidizing the surface of the barrier layer.
 34. Themethod of claim 32, wherein the anode modification layer is formed bydeposition thereof onto the barrier layer.